A variety of double-coated foam tapes are being used for structural bonding in certain applications replacing spot welds, tack welds, or rivets. Such applications include, for example, the bonding of side molding to automobiles, fiberglass body panels to motor homes, plexiglass inspection windows onto equipment cabinets, and the like. The foam layer of these tapes usually has a polymer matrix based on polyethylene, polyurethane, polyvinyl chloride, or polychloroprene. These tapes exhibit poor conformability around curved substrates.
U.S. Pat. No. 4,223,067 to Levens, assigned to Minnesota Mining and Manufacturing Co., describes a method for making conformable foam-like acrylic pressure sensitive adhesive tapes using on web-polymerization technology. In the process, a mixture of monomers and 20 to 65 volume percent glass microbubbles is coated onto a backing sheet and then polymerized to a pressure sensitive adhesive state. The polymerization may be initiated by ultraviolet light or less preferably by heat if the mixture includes a heat-activatible polymerization initiator.
The tapes disclosed by Levens are fairly elastic under briefly applied stresses but exhibit low elasticity under prolonged stress and therefore adhere to rough and uneven surfaces. These tapes exhibit high peel adhesion. The method of Levens, however, requires a long duration to complete polymerization. This makes the tapes expensive to produce. Moreover, coatings having a thickness greater than about 0.2 mm involving neat monomers tend to produce excessive bubbles. If ultraviolet light is used to accomplish polymerization, the composition must be UV transparent. This means that the composition must be free of coloring pigments, or other ultraviolet light absorbing fillers. Also, because the process requires the compositions to comprise a photoinitiator, the compositions tend to yellow over time.
U.S. Pat. No. 4,612,242 to Vesley et al., also assigned to Minnesota Mining and Manufacturing Co., indicated that the white color of the Levens tape, caused by the absence of pigment, made the tape undesirably visible in certain applications, but that the addition of carbon black in an amount sufficient to produce a desirable black appearance would block the ultraviolet radiation from polymerizing the mixture to a pressure-sensitive adhesive state. Vesley et al.'s solution to the problem was to coat the glass microbubbles with an inorganic film, e.g. silver, having a thickness that does not unduly inhibit polymerization.
While the method of Vesley et al. does impart some color to the tapes, it has certain drawbacks. It still takes a long time to effect polymerization, making the tapes expensive to produce. Moreover, glass microbubbles having an inorganic coating are expensive, adding to the cost of producing the tapes. Only a limited number of colors are available in this process.